Insulation system for liquified gas tanks

ABSTRACT

The invention relates to thermally-insulating containers particularly for liquefied gas tankers. It is known to use sprayed polyurethane foam for insulating the cargo tanks, but because of stresses in use load bearing &#34;corners&#34; are required for anchoring the layers. Such corners have previously been provided by the use of frames of balsa-wood. The invention provides the use of sheet material of curved configuration welded along the corners which are designed such that the foam can be applied directly thereto, the arrangement providing corners of sufficient strength for anchoring. The space behind the sheets may be filled with inert gas and can be used for water drainage.

The present invention relates to thermally-insulated containers and hasparticular, although not exclusive, application to the overwatertransportation of liquefied gas, especially liquefied natural gas inocean going tankers.

It is known to transport liquefied natural gas in double-hulled tankersin which self-supporting aluminium alloy cargo tanks are maintained inposition independently of the tanker's hulls by support pads of balsa orplywood bearing on the floor of the inner hull. The inner hull is linedinternally with a layer of polyurethane foam adhered to said hull andcovered with an inner layer of fibrous glass, the inner surface of whichfibrous glass layer is spaced from the cargo tank. The polyurethane foamhad adequate density, stability, strength, impermeability and resistanceto cracking to act as a fluid-tight secondary barrier to protect thehull should the cargo tank spring a leak. However, the foam has asufficiently large coefficient of thermal expansion to tend, when cooledby the presence of liquefied natural gas in the cargo tank, to shrinkaway from the inner hull at angular corners thereof. This problem haspreviously been overcome by providing in said corners, prior toapplication by spraying of the polyurethane layer, ribs of balsa orplywood to provide additional keying surface area for the foam.

Such an application is disclosed in our British patent specification No.1,203,496 and in its patent of addition specification No. 1,301,368. Ithas now been found that said ribs can be replaced at a substantial costsaving (of the order of U.S. $1,000,000 per ship) with "load-bearing"sheet material extending across the respective corners to present aconcavely curved surface to the polyurethane foam layer instead of theangular corner of the inner hull.

By load-bearing as used in this specification, we mean capable ofbearing the load imposed or which might be imposed on the respectivecorner of the internally coated hull (or other container) in use. In thecase of double-hulled tankers of the kind described above, said loadwould be that which could be imposed by leakage of liquefied natural gasfrom the cargo tank.

According to the present invention therefore, there is provided athermally-insulated container having at least one angular corner and aninner lining of thermally-insulating material adhering to said containerwherein load-bearing sheet material extends across said corner below thelining to present a concavely curved surface to the thermally-insulatingmaterial and said material adheres thereto.

As mentioned above, the invention has particular application to theinner hull of a double-hulled tanker of the kind previously referred to.It will be apparent however that it has other applications especially tothe storage of liquefied gas in land tanks in which a liquefied gas iscontained within a storage tank located within a heat-insulatedcontainer. In certain applications, the container of the invention maydirectly contain liquid to be stored or transported.

The nature and identity of the thermally-insulating material will bechosen having regard to the intended use of the container of theinvention. In the case of containers intended to accommodate a liquefiedgas-containing tank, the material will be suitable for cryogenicinsulation such as foamed plastics especially polyurethane. It isespecially preferred that such tank-accomodating containers should belined by spraying with a layer of polyurethane foam which is itselfcovered with a layer of fibrous glass.

The load-bearing sheet material can be of the same or different materialfrom that of the container itself and preferably is resiliently flexibleto accommodate some contraction of the insulating material. Thus, in thecase of mild steel containers, as exemplified by the inner hull ofdouble-hulled tankers of the kind referred to, said sheet materialpreferably is mild steel plate. The sheet material is appropriatelysecured such as by welding to the container at positions spaced from thecorner so that said sheet material extends across the corner in thedesired curved concave configuration. The curvature of said plate, whichusually will be arcuate, is calculated having regard inter alia to theload-bearing requirements and the nature and depth of the insulatinglayer. In the case of corners formed between two walls or wall portions,the sheet material can be of part-cylindrical shape (i.e. having across-section which is a segment of a circle) and may conveniently becut from a cylindrical pipe of appropriate curvature. When the corner isat the junction of three or more walls or wall portions, the sheetmaterial can be part-spherical. In either of said shaped sheet material,the material can extend tangentially from the curved portions thereof tofacilitate welding or other attachment to wall portions adjacent therespective corner.

The space formed between the curved sheet material and the adjacentangular corner of the container can, if desired, be filled with an inertgas or nitrogen and used for water drainage.

The following is a description, by way of example only, and withreference to the accompanying drawings of an embodiment of the presentinvention as applied to a double-hulled tanker for transportingliquefied natural gas. The single FIGURE of the drawing is across-sectional view of a horizontally extending corner formed betweenan upright wall and the base of the inner hull of a double-hulledliquefied natural gas tanker incorporating the present invention.

The liquefied natural gas tanker has a cargo section of double hull,mild steel construction divided by transverse cofferdams into a numberof substantially rectangular holds. The holds are insulated on theirinner surfaces in the manner described below to provide a low cargoboil-off and a liquid-tight secondary barrier protection for the fullheight of liquefied natural gas-containing tanks accommodated therein.Said tanks are fitted one to each hold and are single, divided,self-supporting, flat-sided alumimum cargo tanks. They rest on balsawoodsupports distributed over the base of their respective holds and arelocated by keys fitted at the top and bottom of the holds. Such asupport system is described in our British patent specification No.1,300,730, which is also an addition to British patent specification No.1,203,496.

In accordance with the present invention, each angular corner in theholds is covered by a curved mild steel plate presenting a concavesurface to the inside of the container. In the case of elongate cornersformed between two adjacent walls (including the base and decks) theplate is part-cylindrical and in the case of localized corners formedbetween three adjacent walls (as aforesaid) the plate is part-sphericaland contiguous with the part-cylindrical plates extending to saidlocalized corners. The longitudinal edges of the plates are welded tothe walls. Where necessary the transverse curved edges of the plates arewelded together to completely cover the hold corners. Nitrogen is pumpedinto the space formed between the original hull corners and the saidwelded plates.

Polyurethane foam is sprayed over the inner surfaces of the hold afterwelding said corner plates and fitting the balsawood supports but beforeinsertion of the cargo tank. The foam is applied in several layers,typically 7, inter-spaced with nylon mesh reinforcing, until a depth of6 inches of foam is built up. This layer is then itself covered with twolayers of 3 inch fibrous glass. The dimensions are such that the cargotank when fitted is some 18 inches from the exposed surface of thefibrous glass, although the gap can vary as between the floor, top andwalls of the tankers and their adjacent inner faces of the insulation.

The figure shows in cross-section a typical corner of a hold, being ahorizontally extending corner between an upright wall 1 constituted bythe tankers inner hull or a transverse bulkhead and a base wall 2constituted by the bottom of the tankers inner hull. The corner iscovered by a part-cylindrical 3/16 inch mild steel plate 3 cut from a 9inch radius cylindrical pipe and welded to the walls 1 and 2respectively. The space 4 between plate 3 and the original corner ofwalls 1 and 2 is filled with nitrogen. The walls 1 and 2 and the plate 3are covered as described above with a 6 inch deep layer 5 ofpolyurethane foam which is itself covered with two 3 inch layers offibrous glass 6 and 7 respectively.

The cargo tank 8 is shown in its located position within the cargo hold,the bottom of the tank being supported on balsa pads (not shown), asmentioned hereinbefore, and the side walls thereof being spaced from theadjacent inner faces of the insulation.

It will be appreciated that the invention is not restricted to theparticular description above and that various and numerous modificationscan be made without departing from the invention. In particular, thecorners covered by the load bearing sheet material can be acute orobtuse as well as right-angle as described.

What we claim is:
 1. A thermally-insulating container comprising incombination;a. at least two adjacent metallic walls defining an angularcorner; b. a load-bearing metallic corner plate extending across andspaced from said angular corner, said plate (i) being resilientlyflexible; (ii) having a concavely curved surface; and (iii) beingrigidly attached by welding to the said at least two adjacent wallsalong its edges; and c. an inner lining of a thermally insulatingmaterial which is (i) a foamed plastic (ii) applied directly onto saidwalls and said corner plate.
 2. A container according to claim 1,wherein said thermally-insulating material comprises a polyurethane foamwhich is (i) sprayed in layers (ii) in a known manner (iii) directlyonto said walls and said corner plate.
 3. A container according to claim2 further including at least one layer of fibrous glass covering saidfoam.
 4. A container according to claim 1, wherein said corner plate isformed of the same material as said walls, and is sufficientlyresiliently flexible to accommodate, in use of said container, anythermal movement of the thermally insulating material applied thereto.5. A container according to claim 4, wherein said corner plate comprisespart-cylindrical elongated plates welded along their longitudinal edgesto those corners formed between two adjacent wall portions, andpart-spherical plates welded adjacent each junction of three wallportions, said part-spherical plates being contiguous with saidpart-cylindrical plates, said plates being welded together to define asealed space with said angular corner.
 6. A container according to claim5, and further comprising pump means connected to said sealed space forfilling the latter with an inert gas.
 7. A container according to claim6, and further comprising pump means connected to said sealed space forwater drainage.